New aquisitions: PCjr (minus power supply) and Amstrad PC 6400

From: Ethan Dicks <>
Date: Tue Oct 16 16:47:00 2001

--- Tony Duell <> wrote:
> [...]
> > a nearly complete PCjr PSU).
> To buy or to build? My PCjr was also missing the power supply when I got
> it, so I had to make one.

I did think of that. It's obviously not complicated.
> If you also want to build one, I can tell you that it's about 17V AC, at
> about 50VA-100VA (so say about 3A-5A maximum). The outside 2 pins on the
> power connector are the AC inputs, the middle one is earth ground (and is
> _not_ connected to a centre tap on the transformer winding, no matter
> what some ancient posts here said).

Perfect. Just what I would need to know to make one.

> You can make a PSU...connector... by cutting down a disk drive power
> socket)

I wasn't sure the pins on the back of the PCjr were the right size for
that (there are a couple diameters of Molex pins), but I could try it.
It wouldn't be as nice as a molded connector, but it would work.
> I assume you have the PSU board that fits inside the PCjr (it has the
> power connector and switch on the back edge).

I have that part. The innards are intact. The only thing missing was
a plastic divider between one of the sets of pins on the back (it was
broken off and hanging by a sliver when I picked up the unit). Hardly

I am now debating upgrading the sidecar from 128K to 512K (it involves
removing 16 x 4164 chips, adding low-profile sockets and 16 x 41256 DRAMs
and moving a jumper). If I ever intend to use this, I'll probably do it.
If I don't, I can spend the time elsewhere. I have plenty of chips. The
part I thought was funny was that it's essentially the same core circuit
as the COMBOARD-II - a 74S409, a fist-full of DRAM and some 33Ohm
resistors on the CAS and RAS lines (it took our engineers a bit of work
to realize they needed the in-line resistors to dampen the lines from
ringing and undershoot) I still have multiple kilograms of 74S409 chips
if anyone wants to attempt a classic DRAM design. It would even work
with 256K 30-pin SIMMs, so you don't need to layout a board with DIPs...
Even our final design for the VAXBI used it, but with 44256 chips instead
of 41256 chips to save space.


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Received on Tue Oct 16 2001 - 16:47:00 BST

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